I am a newb as well so don't flame me for trying to help...
But I got that same error a couple of times. I'd get it when I tried to send info to my UNO too quickly and it was still tied up with the previous upload. I also got it when I tried to do upload when my board was still configured for a loop back test and had the reset jumpered. Fortunately I figured out the issue rather quickly (as I was being a dope). That said Double Check your IDE settings and verify the correct micro controller and com port is selected.
Still wankered? Than I would suggest you try the loop-back test as it is a simple means of isolating problems then break the problem further into little bite sized pieces.
For instance... Did you try to remove your uno and upload a sketch? Sounds stupid but it you get the SAME ERROR something is probably tying up your com port ON THE COMPUTER... If you get the expected error saying it can't find the specified port then you can assume the error lies further up the chain on your Uno. Either way you learned something about the problem that you can use to further attack it.
Did it ever work? - If so another thing to ask yourself is what changed? Stupid little details will often house the sneaky little green gremlin and it will typically be so stupid you wouldn't think to look. I call it O2O; obvious to the point of obscurity. LOOK! Assume nothing - Test everything.
I am too new to help much further but I will leave you this advise... When handling issues go slowly and methodically or you will confuse yourself further and get deeper in a hole. Observe what your machine does when it is working IN DETAIL so you then you use that knowledge for comparison when it is NOT working. In other words turn on the verbose mode in preferences.
If nothing else works - Take a nap and try again later.
Hope it helps!